1. Field of the Invention
The present invention relates to polyaddition compounds containing uretdione and carboxyl groups and to a process for preparing them. The invention further relates to methods of preparing polyurethane plastics, especially as crosslinkers for polyurethane (PU) powder coating materials, from these polyaddition compounds. These PU powder coating materials may comprise hydroxyl-containing polymers and compounds which react with carboxyl groups. For example, these PU powder coating materials may include polyepoxides and hydroxyalkylamides.
2. Discussion of the Background
Heat-curable pulverulent compositions obtained by reacting a hydroxyl-containing resin with a blocked polyisocyanate are known, and are in widespread use. Conventional PU powder hardeners include, for example, blocked polyisocyanates such as ε-caprolactam-blocked isophorone diisocyanate (IPDI) adducts. Such adducts are described, for example, in DE 21 05 777, DE 25 42 191 and DE 30 04 876. After being heated in the presence of appropriate hydroxyl compounds, these ε-caprolactam-blocked IPDI adducts provide coatings which combine great ease of handling with good leveling, hardness and elasticity and also good chemical resistance.
EP 0 056 167 describes powder coatings composed of a polyisocyanate hardener with ε-caprolactam-blocked isocyanate groups and acid groups and of a resin which either contains hydroxyl groups or can react to form hydroxyl groups. In additional, it is possible to use polyepoxides, such as triglycidyl isocyanurate (TGIC), which react with carboxyl groups present in the coating system. The powder coatings always possess glossy surfaces.
For a considerable time, there has been increasing interest in powder coating materials which provide a matte surface. The reason for this is predominantly practical in nature. Glossy surfaces require a far greater degree of cleaning than matte surfaces. Furthermore, on safety grounds it may be desirable to avoid strongly reflecting surfaces.
The simplest method of obtaining a matte surface is to mix varying amounts of fillers, such as chalk, finely divided silica or barium sulfate, for example, in the powder coating material in order to obtain the desired matte effect. However, adding fillers may cause deterioration in the film properties of the coating, such as adhesion, flexibility, impact strength, and chemical resistance.
The addition of substances incompatible with the coating material, such as waxes or cellulose derivatives, for example, can provide a matte surface, but slight changes during extrusion may lead to fluctuations in the surface gloss, and therefore the matte effect may not be reproducible.
Matte powder coatings of higher quality are obtained if the matte effect is formed by a chemical crosslinking reaction. For instance, EP 0 104 424 describes matte powder coating materials prepared from a carboxyl-containing, ε-caprolactam-blocked polyisocyanate crosslinker. The isocyanate groups of the crosslinker which are formed upon heating react with the hydroxyl groups of the polymer. The acid groups in the crosslinker react with a polyepoxide.
EP 0 698 629 claims a powder coating material which is composed of a hydroxyl-containing polyester, a polyisocyanate hardener containing blocked isocyanate groups and acid groups, and a β-hydroxyalkylamide. The powder coating surfaces are matte.
A disadvantage common to all these PU powder coating materials, however, is the release into the environment of the blocking agents that are eliminated in the course of thermal crosslinking. When these materials are processed, therefore, it is necessary to take environmental and workplace safety measures. Thus, among other precautions, the outgoing air is cleaned or the blocking agent is recovered. This fundamental disadvantage is overcome by the use of PU powder coating crosslinkers which are free from blocking agents, and contain uretdione groups.
Crosslinkers which can be used as crosslinkers for glossy or matte PU powder coatings, based on uretdiones and certain amines, are known. DE-A 195 46 750 describes hardeners for producing polyurethane (PU) coatings with glossy surfaces which are reaction products of isophorone diisocyanate uretdione and disecondary diamines.
DE-A 196 39 844 describes matte PU powder coating materials comprising isophorone diisocyanate uretdiones containing urea groups as a hardener component. These compounds are prepared by reacting uretdiones with water. Intermediates which form during this reaction, with the elimination of carbon dioxide, are primary amines, which react with remaining isocyanate groups to form ureas. In each case, the nitrogen atoms of the urea groups are monosubstituted. Reactions of polyisocyanates with water are difficult to reproduce, owing to the formation of by-products.
DE-A 196 37 375 describes PU powder coating materials which are formulated for a matte effect by means of hardeners comprising isophorone diisocyanate uretdiones containing urea groups. These hardeners are formed by the reaction of isophorone diisocyanate uretdione with disecondary diamines. The disecondary diamines are reaction products of diprimary diamines and maleic or fumaric esters.
DE-A 196 37 377 and DE-A 198 16 547 describe polyaddition products containing uretdione groups and urea groups as hardener components for preparing matte PU powder coating materials. These hardeners are prepared by reacting isophorone diisocyanate uretdione with diamines containing a primary amino group and secondary amino groups. Preparation of the pure diamines is complex and extremely costly.
All of these blocking-agent-free PU powder coating materials possess the disadvantages of providing coatings having poor flexibility and being prepared from relatively expensive precursors.
It is therefore an object of the present invention to provide blocking-agent-free crosslinkers prepared from polyaddition compounds containing uretdione groups, which can be processed in combination with hydroxyl-containing polymers and, if desired, compounds which react with carboxyl groups, such as polyepoxides or hydroxyalkylamides, to give PU powder coating materials whose coatings exhibit good mechanical properties and whose surfaces can be made matte or glossy in accordance with the intended application.
It has surprisingly been found that polyaddition compounds which contain uretdione groups as well as carboxyl groups as well achieve this object.